Certain rhodium based catalysts are known to be particularly suitable for the selective hydrogenation of the carbon-carbon double bonds in unsaturated acrylonitrile-butadiene rubber.
For example, U.K. No. 1,558,491 teaches the use of chlororhodium tris (triphenylphosphine), RhCl(PPh.sub.3).sub.3, in a process to hydrogenate acrylonitrile-butadiene rubber.
Similarly, U.S. Pat. No. 4,464,515 teaches the use of hydrido rhodium tetrakis (triphenylphosphine) catalyst, HRh(PPh.sub.3).sub.4, in a process to selectively hydrogenate unsaturated acrylonitrile-butadiene rubber.
In both of these known processes, the unsaturated nitrile rubber is dissolved in a suitable solvent to provide a viscous rubber solution. The catalyst is then dissolved in the rubber solution. These hydrogenation processes are said to be homogeneous because the substrate and catalyst are contained in the same phase.
The selectively hydrogenated nitrile rubber which is produced by either of the above homogeneous hydrogenation processes is less susceptible to heat-induced degradation in comparison to unsaturated nitrile rubber. One advantage of the above homogeneous processes is that they are efficient with respect to the minimal amount of catalyst which is required to catalyze hydrogenation of the carbon-carbon double bonds. However, a typical disadvantage of any homogeneous catalytic process is that it is generally quite difficult to remove the catalyst from the system when the reaction is completed (in comparison to a heterogeneous process, where the catalyst is not dissolved and hence may be readily removed by filtration or centrifugation).
For reasons of process economy and product performance purity, it is extremely desirable to remove the rhodium hydrogenation catalyst residue from solutions of partially hydrogenated nitrile rubber. Thus, the object of the present invention is to provide a method to treat a rhodium containing solution obtained from the hydrogenation of nitrile rubber.
The present invention provides a method to treat a rhodium containing solution obtained from the catalytic hydrogenation of nitrile rubber, said method comprising:
(i) adding to said solution an additive selected from: PA1 (ii) separating the precipitate from the solution.
(a) thiourea PA2 (b) polysaccharide, and PA2 (c) activated carbon, thereby forming a rhodium-containing precipitate which is not soluble in said solution, and